Microstructured TiO₂ coatings by anodization of Ti6Al4V with oxalic acid

Natural oxidation of Titanium allows the formation of a TiO₂ coating that is known to present relevant tribological and photocatalytic properties. These aspects can be enhanced with nano-structural tailoring of the TiO₂ surface, which can be achieved by electrochemical anodization. This work proposes the use of oxalic acid as an alternative for the anodization of Ti6Al4V with a lower associated risk than fluoride-based electrolytes. Specimens were anodized in an oxalic acid aqueous solution at different voltages and times. Results indicate that anodization at 30 V allows the formation of a porous and amorphous TiO₂ coating with a thickness of approximately 5 μm. Increased anodization time does not seem to impact the thickness of the coating. However, it promotes the deposition of stacked-flakes structures on top of the oxide layer, attributed to the formation of titanium oxalates. Subsequent heat treatments at 500 °C for 2 h caused the crystallization of the amorphous TiO₂ and the decomposition of the oxalate into TiO₂, while retaining the morphology. The transformation is confirmed by Raman spectrometry and X-ray diffraction. Compared with untreated Ti6Al4V, specimens anodized for 0.5 h present an increase in microhardness of 27% and a reduction in the average coefficient of friction against 100Cr6 steel of approximately 17% after stabilization. The proposed method allows obtaining a nanostructured TiO₂ coating with a fluoride-free electrolyte with properties comparable to other methods. Graphical abstract: [Figure not available: see fulltext.]